1. Field of the Invention
The present invention relates an internal combustion engine, and particularly to improved technology with respect to engine cooling for an air-cooled internal combustion engine for a motorcycle.
2. Description of the Background Art
A known internal combustion engine, and particularly an air-cooled internal combustion engine for a motorcycle, includes a plurality of large cooling fins at an outer peripheral surface of a cylinder block and a cylinder head in order to improve cooling efficiency of the engine. These cooling fins are formed of thin plates creating a large surface area in order to improve cooling efficiency. It is therefore easy for these cooling fins to vibrate. A well-known means for preventing vibration of the cooling fins comprises insertion of elastic members, such as vibration preventing rubber or the like, between the cooling fins. Use of the elastic members between cooling fins is disclosed in Japanese Patent No. 2791896 (page 2, FIG. 2) and Japanese Utility Model Laid-open No. Sho. 59-43648 (page 1, FIG. 1).
In the invention disclosed in Japanese Patent No. 2791896 and Japanese Utility Model Laid-open No. Sho. 59-43648) and shown in present FIG. 9 and FIG. 10, there are respectively disclosed, in an internal combustion engine, and particularly in an air-cooled internal combustion engine for a motorcycle, provision of a plurality of cooling fins OF3 on the outer peripheral surface of a cylinder head 03 of the engine, extending parallel a specified length towards the outside from the outer surface, as cooling measures for the internal combustion engine. A structure is disclosed where elastic bodies such as vibration control rubber OR, having an external shape that is trapezoidal or a substantially square column, is press fitted between cooling fins 0f3, 0f3, of the cooling fins OF3, that are respectively opposite, in order to control vibration of these cooling fins OF3.
However, with the inventions disclosed in Japanese Patent No. 2791896 and Japanese Utility Model Laid-open No. Sho. 59-43648) above, the elastic member such as vibration control rubber press fitted between the cooling fins facing each other has an external shape that is trapezoidal or a substantially square column, and insertion of an elastic member of such as shape between cooling fins is a main cause of a flow separation phenomenon of traveling wind in the cooling fins. This flow separation phenomenon disrupts flow of traveling wind, and inhibits the smooth flow of traveling wind, which means that in order to obtain sufficient cooling capability for an internal combustion engine there remains a problem that a cooling unit is large in size.
Another well-known technique for improving cooling efficiency of a cylinder block includes a structure where central ribs projecting from the upwind side to a substantially central part of each cylinder and connecting upper and lower cooling fins are provided at an upwind side of the cylinder block. In this configuration, cooling wind flows laterally at the central ribs and is caused to pass through wind passing holes, so as to sweep to the rear, preventing muffling and stagnation of heated air around the cylinder block. An example of this configuration is disclosed in Japanese published Utility Model No. Sho 63-29161 (page 2, FIG. 1 and FIG. 2). It is also well known to align a plurality of sideways cooling fins and vertical guidance fins about the cylinder head to improve cooling efficiency. This feature is disclosed in Japanese Utility Model Laid-open No. Sho. 55-92022 (page 1, FIG. 3).
The invention disclosed in Japanese published Utility Model No. Sho 63-29161 described above and shown in present FIG. 11 discloses an engine structure, in a multiple cylinder air-cooled internal combustion engine, at an upwind side of a cylinder block 02, where central ribs 0F projecting to an upwind side and connecting with vertical cooling fins OF2 are provided at substantially central parts of each cylinder, cooling air therefore flows laterally to the central ribs 0F, the cooling air passes air passing holes without stagnating, spreads to the rear and thus improves cooling efficiency.
Further, the invention disclosed in Japanese Utility Model Laid-open No. Sho. 55-92022 disclosed above and shown in present FIG. 12 discloses, in a double overhead cam (DOHC) 4-cycle internal combustion engine having cooling air arriving from the front, provided with a cam cover 04 at an upper rear part of a cylinder head 03, a cooling structure having a plurality of horizontal cooling fins OF3 projecting in a horizontal direction provided on a cylinder head 03, and vertical guidance fins OF facing from a front side end of the horizontal cooling fins OF3 to a rear cover, to bring about improved cooling efficiency of the cylinder head 03.
In the inventions disclosed in Japanese published Utility Model No. Sho 63-29161 and Japanese Utility Model Laid-open No. Sho. 55-92022 above, it is intended, in addition to improving the external rib structure and cooling fin shape structure of the cylinder section, to improve cooling efficiency of the cylinder section by controlling flow of cooling air, and although it is possible to achieve satisfactory cooling, the structure of the resulting cylinder section becomes very complicated, and cost of the engine is increased.
In the above-described circumstances, there is a need to provide a lower cost engine, and to provide a cooling structure for an air-cooled internal combustion engine capable of achieving extremely effective cooling of a cylinder section by adopting simple improved technology without adding separate structural improvement to the structure of the cylinder section.